The present disclosure relates generally to switching mode power supplies (SMPSs), more particularly to SMPSs capable of performing high-voltage charging while maintaining their operating voltages and the relevant control methods.
Universal Serial Bus (USB) is one of the communication interfaces most broadly used in daily life nowadays. Beside its reliable, rapid data transmission, USB also provides limited power to the peripherals connected to it. Most mobile phones are charged using USB chargers, for example.
To make USB more suitable for powering various electric apparatuses and reducing the number of power cables needed, USB Implementers Forum Inc., a non-profit corporation founded by the group of companies that developed the USB specification, has announced USB Power Delivery (PD) to enable the maximum functionality of USB by providing more flexible power delivery along with data over a single cable. USB PD offers increased power levels from existing standards up to 100 W, so it is possible to enable new higher power use cases such as USB powered hard disk drivers and printers.
USB PD requires a USB charger having its output voltage variable in a range from 5V to 20V, and this range could be expanded as broad as being from 3V to 20V in the future. So far, the voltage ratings of a USB charger complying USB PD are 5V, 12V and 20V. If a USB charger employs isolation topology, meaning no direct-current (DC) connection between the output voltage of the USB charger and the power grid powering the USB charger, the operating voltage VDD powering an integrated circuit at a primary side is normally generated by rectifying an induced voltage across an auxiliary winding of a transformer. The induced voltage of the auxiliary winding, as it is named, changes following the change to the output voltage VOUT, so does the operating voltage VDD. Conventional integrated circuits normally sustain operating voltage VDD up to about 30V, and need operating voltage VDD more than 10V to drive a power switch. If it is configured that the output voltage VOUT is 20V to induce the operating voltage VDD of 30V, then, when the output voltage VOUT is 5V, which is the minimum voltage rating defined by USB PD, the operating voltage VDD will be as low as 7.5V, probably too low to power an integrated circuit.
To solve the problem mentioned in the previous paragraph, FIG. 1 provides a conventional USB charger 10, including a transformer with a primary winding PRM, a secondary winding SEC, and an auxiliary winding AUX, all inductively coupled to one another. An alternating-current voltage from a power grid is rectified by a bridge rectifier 12 to generate an input voltage VIN at power node IN. Resistor RHV is connected between power node IN and high-voltage node HV, providing power controller 18 the current required for high-voltage startup.
Power controller 18, an integrated circuit at the primary side, provides PWM signal SDRV to drive power switch 20, which equivalently chops input voltage VIN to generate alternating-current voltage VSEC across secondary winding SEC. By rectifying alternating-current VSEC output voltage VOUT is generated at output node OUT to power load 24, which for example is a mobile phone or a set of batteries connected to an USB port (not shown).
Operational amplifier 22 compares output voltage VOUT with target voltage VTAR, and its output, via photo coupler 26, controls compensation voltage VCOMP at compensation node COMP, based on which power controller 18 controls the duty cycle and the switching frequency of PWM signal SDRV. Accordingly, USB charger 10 employs a close loop to regulate output voltage VOUT at about target voltage VTAR. Target voltage VTAR is set to be 20V if the voltage rating of USB charger 10 is 20V, and it is switched to become 5V if the voltage rating is changed to be 5V.
USB charger 10 has a low drop out linear regulator (LDO) 16, connected in series with rectifying diode DAUX between power node VDD and a terminal of auxiliary winding AUX. Operating voltage VDD at power node VDD performs as a power source to power power controller 18. LDO 16 clamps operating voltage VDD, making it not more than the maximum operating voltage that power controller 18 can sustain. In light of system configuration, when output voltage VOUT is 5V, an induced voltage VAUX of auxiliary winding AUX could be designed to be about the minimum operating voltage required for power controller 18 to operate normally, and this minimum operating voltage is 10V for example. Meanwhile, LDO 16 seemingly functions as a short circuit, making operating voltage VDD about 10V, just high enough for the normal operation of power controller 18. In case that output voltage VOUT is 20V, induced voltage VAUX becomes 40V, so LDO 16 consumes 10V drop to clamp operating voltage VDD at 30V, preventing power controller 18 from damage of over-voltage stress.